This invention relates to a process for the preparation of a flexible foam and of a molded flexible foam. This process comprises reacting a liquid, storage stable, allophanate modified polyisocyanate having an NCO group content of from about 20 to about 42%, with an isocyanate-reactive component, in the presence of a blowing agent, and at least one catalyst, wherein the quantities of polyisocyanate and isocyanate-reactive component are such that the NCO index of the system is from about 70 to about 130. The present invention also relates to the flexible foams and molded flexible foams prepared by these processes.
Flexible polyurethane foams and processes for their preparation are known and are described in, for example, U.S. Pat. Nos. 4,478,960, 4,833,176,4,876,292, 4,945,117, 5,070,114, 5,369,138, 5,521,225 and 5,521,226, and in European Patents 0010850 and 0022617. Mixtures of diphenylmethane diisocyanate (MDI), polyphenylene polymethylene polyisocyanates, prepolymers based on such materials, and toluene diisocyanate are known to be suitable for preparing flexible foams. See, for example, U.S. Pat. Nos. 4,239,856, 4,256,849, 4,261,852 and 4,365,025.
U.S. Pat. No. 5,070,114 discloses the preparation of flexible foams prepared from diphenylmethane diisocyanate (MDI) based prepolymers having a rather low NCO value and from an isocyanate-reactive composition comprising relatively high amounts of water.
U.S. Pat. No. 4,478,960 describes the preparation of flexible polyurethane from 1) a prepolymer based on MDI and optionally polymeric MDI, and a polyol having from 5 to 30% by wt. of oxyethylene groups, wherein the prepolymer has an NCO group content of 12 to 30% by wt., 2) polymeric MDI, if 1) above is MDI, 3) a polyol having a low EO content and 4) a blowing agent.
A process for the preparation of a flexible foam from a prepolymer having an NCO content of from 15 to 30% by wt. is disclosed by U.S. Pat. No. 4,945,117. These flexible foams are prepared from a polyol having a functionality of at least 4.
The preparation of flexible foams from polyisocyanate prepolymers is disclosed by U.S. Pat. No. 5,369,138. The prepolymer is reacted with water and a mixture of a polyol having a low oxyethylene content and a polyol having a high oxyethylene content.
U.S. Pat. No. 4,876,292 describes a particular isocyanate-reactive mixture suitable for preparing flexible polyurethane foams. This mixture comprises up to 3 parts by wt. of an amine corresponding to a specific formula, up to 30% by wt. of a polyoxyalkylene polyamine having a molecular weight of from about 400 to about 5000 and containing from 2 to 3 primary amino groups, and from 70 to 100% by wt. of one or more polyether polyhydroxyl compounds having hydroxyl functionalities of from 2 to 3 and molecular weights of from 1000 to 10,000. Polymethylene poly(phenyl isocyanates) or prepolymers thereof are suitable for reaction with these isocyanate-reactive mixtures.
The process for producing flexible foams disclosed by U.S. Pat. No. 5,521,225 comprises reacting a polyisocyanate composition having an NCO group content of 10 to 25% by wt., with a specific polyol composition. Suitable polyisocyanate compositions comprise a) an isocyanate prepolymer having an NCO content of 5 to 15% by wt. and being prepared by reacting an excess of polyisocyanate and a polyol of specified functionality, equivalent weight and EO content, and b) a polyisocyanate having an NCO content of 30 to 33% by wt.
Allophanate modified isocyanates are also known in the art. Various isocyanates containing allophanate groups and processes for their production are disclosed in, for example, U.S. Pat. No. 4,738,991, 4,866,103, 5,319,053 and 5,319,054, European Patents 0,031,650 and 0,393,903, and GB Patent 994,890. European Patent 0,393,903 is of relevance; it discloses some examples wherein flexible foams are prepared. These flexible foams, however, are produced from an allophanate-modified isocyanate based on MDI containing 20% by weight of the 2,4'-isomer of MDI, and high molecular weight polyether polyols having a functionality of 2 to 3.
U.S. Pat. No. 4,738,991 is also of interest in that it discloses one example wherein a flexible foam is prepared. This flexible foam, however, is produced from an allophanate-modified isocyanate prepared from toluene diisocyanate and ethylene glycol. It has since been found that at the high levels of TDI allophanate modification which are necessary to minimize the polymer polyol content in the foam formulation, a TDI allophanate prepared from ethylene glycol gives only poor humid aged compression set.
European Patent 0,031,650 describes MDI based allophanate modified isocyanates. These blends, however, consist of an allophanate modified MDI which is the reaction product of an alcohol with an MDI based isocyanate comprising more than 20% by weight of the 2,4'-isomer of MDI. It also discloses that these may be suitable isocyanates for flexible foam molding.
U.S. Pat. No. 5,874,485 disclosed flexible foams and flexible molded foams based on allophanate-modified diphenylmethane diisocyanates, and processes for the production of these foams. The allophanate-modified diphenylmethane diisocyanates are characterized by an NCO group content of from 12 to 32.5%, and are prepared by reacting an aliphatic or aromatic alcohol with diphenylmethane diisocyanate of the specified isomer distribution. These allophanate-modified diphenyl-methane diisocyanates essentially correspond to those described in U.S. Pat. No. 5,319,053. Flexible foams and flexible molded foams can be prepared by reacting these allophanate-modified diphenylmethane diisocyanates with an isocyanate-reactive component in the presence of a blowing agent and at least one catalyst. To lower compression sets of the flexible foams, it was necessary to blend the allophanate-modified MDI with PMDI. Compression sets low enough for the foams to be useful in automotive seating, etc. could be obtained by the addition of an amine based alcohol crosslinker such as diethanolamine.
GB 1,520,846 discloses flexible foams and a process for the production of flexible foams from a liquid polyisocyanate prepared by reacting an excess of TDI in the absence of an allophanate catalyst, with an oxypropylated triol having a molecular weight of 200 to 800.
Flexible foams are also disclosed by U.S. Pat. No. 4,554,295. These flexible foams comprise the reaction product of unmodified toluene diisocyanate with an isocyanate-reactive component comprising 100% by weight of a polymer polyol. These flexible foams require a minimum quantity of polymer polyol to be used as the isocyanate-reactive component in order to attain a certain hardness in the foams.
It has presently been found that allophanate modified isocyanates based on 1,2-propylene glycol or 1,3-butanediol can build hardness (ILD) in flexible foams and flexible molded foams using minor amounts or even no polymer polyol. Physical properties are suprisingly reasonable. Modified toluene diisocyanates usually result in poor humid aged compressions sets but the 1,2-propylene glycol and 1,3-butanediol allophanate modifications give acceptable values. Hardness of the resultant foams can be varied by simply varying the isocyanate index of the reaction system.
Flexible foams of varying hardness, reasonable physical properties and good humid aged compression sets can be formulated without polymer polyols by using allophanate modified polyisocyanates based on 1,2-propylene glycol or 1,3-butanediol. The advantage of this is a lower cost system due to the elimination of polymer polyols which are more expensive than conventional isocyanate-reactive components. In addition, prior art systems require the presence of a third stream for the polymer polyol component in order to easily adjust the hardness of the resultant flexible foam. The hardness of the foams can be adjusted by varying the isocyanate index.
Another advantage offered by the present invention is better demold characteristics of the flexible foams and flexible molded foams in comparison to convention flexible foams and flexible molded foams based on toluene-diisocyanate (TD-80). This is determined by compressing the four (4) corners of a freshly demolded block and measuring the permanent set.